Pressure-swing adsorption is a technique which has gained increasing interest in recent years because it eliminates the need for thermal regeneration of an adsorber bed.
In pressure-swing adsorption, a gas mixture is passed through an adsorber bed which selectively retains a component of the gas mixture at a relatively high adsorption pressure which, although higher than a desorption pressure, may nevertheless not have a magnitude significantly greater than atmospheric pressure. Following the adsorption phase, which generally is carried out until just prior to breakthrough of adsorbed component, the adsorber is flushed in the flushing or purging phase with a purging gas, generally at the adsorption pressure. Subsequently the adsorber is subjected to a pressure reduction which may release a purging or flusing gas and then a product gas rich in the adsorbed component. This desorption phase is effected at a pressure which is below that of the adsorption phase.
Generally a number of such adsorbers, usually using molecular sieve adsorbents, will be provided and operated in a phase-shifted relationship so that communication between two adsorbers in different phases can allow pressure rebuilding in a desorbed adsorber in a pressure buil-up phase and at the same time the reduction of pressure in an adsorber in which adsorption has terminated. The generation of the flushing gas can be effected in a similar manner.
An expansion phase can follow the adsorption phase and precede the flushing and desorption phases.
Consequently a large number of adsorbers can be provided and the plant formed with the necessary valves and pipes interconnecting the adsorbers and which, when opened and closed, communicate between the adsorbers, with the product, waste gas and inlet gas lines, and with compressors and vacuum pumps which may be provided.
Oxygen having a purity of at least 99.5 vol. percent is required for a multitude of applications in the practice, e.g. in welding for gas cutting, or in medicine.
A common method of producing oxygen is the cryogenic rectification (fractionation) of air. This process is carried out at considerable capital cost so that from an energy point of view its application is interesting only in plants with high outputs.
When smaller consumers are concerned, oxygen is usually supplied in liquid form and stored, being drawn as needed via a cold water gasifier. This supply technique has, however, the drawback for the customer that the price of oxygen depends largely on transport costs, a factor especially relevant in countries with little industrialization. Moreover, such a supply can easily be jeopardized by, for example, bad traffic conditions, political instability etc.
German patent document--open application DE-OS No. 28 55 626 discloses a method for producing oxygen with a purity of above 99.5 vol. percent from gas mixtures containing N.sub.2, O.sub.2 and Ar.
This method is carried out in a pressure-swing adsorber system with carbon molecular sieves.
Preferably, the adsorption unit includes two adsorbent beds whose working cycles each comprise an adsorption phase and a desorption phase. The working cycles in both adsorbent beds are dephased relative to one another.
The adsorbent beds which are filled with the carbon molecular sieve provide during the desorption phase an intermediate product that is enriched with oxygen and depleted of argon by comparison to the supplied N.sub.2 /O.sub.2 /Ar gas mixture.
Thereafter the intermediate product is subjected to zeolitic adsorption in an adsorption unit which also preferably includes two adsorbent beds and in which nitrogen is preferentially removed (while argon is not adsorbed). When this method is carried out with a dry and carbon-dioxide-free air, oxygen is produced with a purity of 99.7 vol. percent during the adsorption phase of the zeolitic adsorption unit.
Since the drying of the air is only optional and frequently is omitted, the amount of produced waste gases is greater at the cost of a smaller amount of product gas.
Moreover, the fractionation of the gas obtained during the desorption in the carbon adsorption unit requires two intermediate reservoirs for this gas. A further drawback of the known method resides in the necessity for large-volume adsorbers since the method is operated at normal pressure. Finally, the prior-art method requires the use of a vacuum pump for each stage, i.e. for the carbon adsorption unit and the zeolitic adsorption unit, thus rendering the method complicated and frequently uneconomical.